Real-time monitoring of biomass during Escherichia coli high-cell-density cultivations by in-line photon density wave spectroscopy.

An efficient monitoring and control strategy is the basis for a reliable production process. Conventional optical density (OD) measurements involve superpositions of light absorption and scattering, and the results are only given in arbitrary units. In contrast, photon density wave (PDW) spectroscopy is a dilution-free method that allows independent quantification of both effects with defined units. For the first time, PDW spectroscopy was evaluated as a novel optical process analytical technology tool for real-time monitoring of biomass formation in Escherichia coli high-cell-density fed-batch cultivations. Inline PDW measurements were compared to a commercially available inline turbidity probe and with offline measurements of OD and cell dry weight (CDW). An accurate correlation of the reduced PDW scattering coefficient µ s ' with CDW was observed in the range of 5-69 g L -1 (R 2  = 0.98). The growth rates calculated based on µ s ' were comparable to the rates determined with all reference methods. Furthermore, quantification of the reduced PDW scattering coefficient µ s ' as a function of the absorption coefficient µ a allowed direct detection of unintended process trends caused by overfeeding and subsequent acetate accumulation. Inline PDW spectroscopy can contribute to more robust bioprocess monitoring and consequently improved process performance.